The purpose of an Automatic Air Collision Avoidance System (ACAS) is to prevent collisions in the air between aircraft, each having the system. A fly out is an automatic maneuver per-formed to avoid collision with another aircraft. Once the fly out maneuver is activated, maneuvers ordered from the pilot are disregarded. Each aircraft with the system continuously computes an escape angle and load factor to be used by the aircraft during a fly out maneuver for the case of an approaching collision. The escape angle is a relative roll angle. At the same time the aircraft computes optimized fly away paths in the air. The fly away paths are computed by means of an aircraft response model. The fly away path is a prediction of the space within which the aircraft will be located with a certain probability if a fly out maneuver will occur.
The fly away path is a trajectory in the air surrounded with a cone shaped space. The size of the cone shaped space surrounding the trajectory depends on uncertainties in the prediction of the fly away path. The uncertainties in the prediction is for example due to inaccuracy of the aircraft response model, timing accuracy of fly out activation due to inaccuracy in the assumption of when the fly out maneuver will begin, and last instant maneuvering. The computed fly away path is sent to the other aircraft. When the other aircraft receive a fly away path, the path is booked. Thus, the booked fly away paths are known to all neighboring aircraft having the system.
The aircraft continuously receives fly away paths from the other aircraft. The system detects an approaching collision based on the own fly away path and the fly away paths received from the other aircraft, and upon detecting an approaching collision activates the automatic fly out maneuver. During the fly out maneuver the aircraft is ordered to take the escape angle and load factor computed at the same time as the last booked fly away path. A collision is detected when the system detects that the own booked fly away path crosses a booked fly away path of another aircraft. The fly out maneuver should occur within a booked space that is known to the other aircraft. If no collision is detected the system computes a new escape angle and load factor to be used during a fly out maneuver, and a new fly away path based on the fly away paths received from the other aircrafts. The new fly away path is sent to the other aircraft.
A problem in connection with automatic air collision avoidance systems is that in some situations the uncertainties in the pre-diction are large. To make sure that the fly out maneuver is within the booked space of the fly away path, the width of the booked path is increased. When the width of the booked path is increased, the risk for nuisance is increased. With nuisance is meant an event, which results in an unintentional or unpredicted response or activation of the system. When the risk for unintentional or unpredicted activations of the fly out maneuver be-comes too high, the system for automatically avoiding collisions is turned off and by then the availability of the system is de-creased.
One of the factors affecting the uncertainties are last instant maneuvers of the aircraft during the time delay between computing a fly away path and receiving and booking the fly away path by the other aircraft. It takes approximately 0.3 s from computing the fly away path until the other aircraft have received the path. This problem is partly taken care of in known automatic air collision avoidance systems, by performing a prediction of aircraft movements during the following 0.3 s, and considering this prediction when computing the fly away path. This prediction assumes that the pilot maneuvers during the next 0.3 s are the same as in the prediction moment, i.e. that the position of the control stick is fixed. However, this is not always the case, on the contrary the pilot is free to perform maneuvers as he desires until an approaching collision is detected and the fly out maneuver is started. Particularly, there is a problem if the pilot performs maneuvers that counteract the booked fly away path during said time delay. In a worst case it may no longer be possible for the aircraft to follow the booked fly away path, due to the pilot maneuvers.